High frequency electronic circuits, such as RF circuits or microwave circuits, comprise a number of electrical devices that are connected together by a transmission line structure that routes high frequency electrical signals to each device. The transmission line structure may take a variety of forms such as, but not limited to, coaxial, waveguide, stripline and microstrip transmission lines. Many other transmission line structures are possible and well known to those skilled in the art. Useful texts on this subject include “Field Theory of Guided Waves”, IEEE Press, ©1991 by R. E. Collin; “Microwave Engineers' Handbook”, Artech House, ©1971 compiled and edited by T. S. Saad; and “Microwave Filters, Impedance Matching Networks, and Coupling Structures”, McGraw-Hill, ©1964, by G. L. Matthaei, L. Young, and E. M. T. Jones.Each transmission line structure usually consists of metallic conductors and a low-loss dielectric.
The electrical devices in the high frequency circuits may be, for example, a filter or a circulator and the like. These electrical devices generally extract a desired signal from an electrical signal and routes the energy corresponding to this desired signal to a transmission line. However, many circumstances may cause a reflection of the energy back to the electrical device from where it came which may damage the electrical device. These circumstances include impedance mismatch, faulty components, incorrect switch settings, or improper operating frequency. To prevent damage, high frequency electronic circuits incorporate electrical devices called terminations to absorb unwanted electromagnetic energy. The absorbed electromagnetic energy is converted to heat. To be effective, these terminations must provide adequate power absorption and reflect incoming electromagnetic energy as little as possible. This requires any impedance discontinuity between a transmission line structure and a termination to be small.
The terminations must also be able to transfer the heat generated by a termination due to energy absorption from the termination to the environment to ensure that the total absorbed energy will not produce excessive temperatures within the termination. In general, there are three methods for removing heat from the termination: convection, radiation, and conduction. For many applications, such as a space environment, convection and radiation are ineffective leaving conduction as the only effective method of heat removal. For heat to be removed by conduction, the termination must be mounted to a thermally conductive surface. The portion of the environment that accepts the transferred heat is known as a heat sink. The heat sink may be a surface exposed to a cooling fluid such as water or moving air. However, where conduction dominates, the heat sink consists of the thermally conductive surface.
The thermal design of the termination now involves effectively transferring the heat from the absorber to the heat sink in order to minimize the temperature rise within the termination. A large temperature rise within the termination may cause physical damage to the materials of the termination. Furthermore, the performance of the termination deteriorates as the temperature sensitive components within the termination reach ever higher temperatures.
The high temperatures can be mitigated by increasing the size of the termination. However, in some applications such as space, the components may have size limitations. The heat generated by a termination also produces a high temperature in the vicinity of the termination that, in many applications, can seriously complicate the thermal design of nearby components. Consequently, terminations are frequently located remotely from its associated equipment and connected by coaxial transmission lines.
Current terminations with a coaxial interface include resistive film chip terminations and absorptive terminations. Resistive film chip terminations are mainly used with microstrip transmission line structures. Resistive film chip terminations comprise a thin resistive film that is deposited onto a thermally conductive dielectric that is usually mounted on a copper carrier. The electromagnetic energy is provided to the resistive film, which heats up thereby dissipating the electromagnetic energy. However, the generated heat is concentrated in a small area, which creates a high thermal flux density and a high thermal stress on the resistive film. Furthermore, there are multiple interfaces, which creates a long thermal path to the mounting surface that acts as a heat sink for the resistive film chip termination. Consequently, a high internal temperature develops, which affects the structural integrity of the resistive film.
To reduce the temperature within the resistive film chip termination, the area of the resistive film may be increased, or the thickness of the thermally conductive dielectric may be reduced. However, this increases the capacitance of the resistive film chip, which adversely affects performance at microwave frequencies and limits the usefulness of resistive film chip terminations in high power applications. Alternatively, a “distributed” termination may be created using multiple resistive film chip terminations connected by a power splitting network. However, such a distributed termination is complicated, less reliable, and physically large.
In absorptive terminations, the dissipation of the electromagnetic energy occurs in a lossy dielectric material, which absorbs the electromagnetic energy. The absorbed electromagnetic energy is converted to heat. Examples of lossy dielectric materials that are used in current absorptive terminations include'silicon carbide or an epoxy loaded with an iron powder. Absorptive terminations are more effective than resistive film chip terminations for higher power applications since they can be designed to have a lower thermal flux density.
However, prior art absorptive terminations have a somewhat inflexible design. The coaxial structure limits the ability to spread the heat since adjusting the absorption within the termination requires component geometries that are complicated to fabricate and assemble. In addition, prior art absorptive terminations generate heat that is some distance away from the mounting surface that provides a heat sink for dissipating the generated heat. This long thermal path leads to higher internal temperatures within the termination.